The present invention relates to a new and improved method of fire detection and an improved construction of fire detection installation.
Generally speaking, the method for fire detection and the fire detection installation of the present development rely upon an arrangement of the type comprising at least two sensor or feeler elements possessing different operating or functional principles and a common evaluation circuit for the evaluation of the changes in the conditions of the sensor elements and for triggering or giving an alarm. Preferably, one of the sensors is a photoelectric transducer and the second sensor either comprises a second photoelectric transducer operating according to a different principle or an air-accessible ionization chamber.
Fires which cause damage and arise in practice can be basically classified into two categories, namely smoldering fires and open fires i.e. those with licking or uncontrolled flames. In the case of smoldering or slowly burning fires there is further differentiated between the so-called pyrolytic fires and glow fires. Pyrolytic fires require for the sustaining thereof a continuous infeed of energy in the form of heat, whereas the glow fires, once ignited, automatically further develop.
Depending upon the nature of the fire it is necessary to undertake different counteractive or fire fighting measures. In the case of smoldering fires life-saving measures take priority, since, as a general rule, there has not yet arisen any considerable property damage and because of the impending danger of suffocating or becoming overcome by smoke inhalation all endangered human beings or occupants must be encouraged to depart from the endangered area or region. On the other hand, in the case of fires having open or licking flames of predominant importance is extinguishing the fire, since considerable property damage will result, and frequently the saving of human life necessitates resorting to outside assistance. The ever growing population density and the increasing accumulation of valuable personal properties in buildings make it more and more desirable to cut down the time until there are implemented the requisite counteractive or fire fighting measures. Furthermore, the access of fire fighting equipment to the site of the fire, in many instances, is markedly hindered by traffic conditions, resulting in the fire fighting personnel frequently arriving too late at the site of the fire. Therefore, it becomes increasingly more important that the alarms which are received from the fire alarm or fire detection equipment enable recognition of the type of fire which is being encountered.
The heretofore known automatic fire alarm installations generally contain a sensor or feeler element which responds to combustion phenomena as well as an evaluation circuit which evaluates the electrical signal delivered by the sensor element for alarm signalling or triggering purposes.
Most of the heretofore known fire alarm installations give or trigger an alarm whenever the signal delivered by the sensor element exceeds a set threshold value, or there is possibly evaluated the change as a function of time of the sensor signal (differential fire alarm). It also has already been proposed to combine a number of sensors having different functional principles in a detector, in order to increase the sensitivity and to reduce the danger of false alarms. In German Pat. No. 2,452,839 there has been proposed a fire alarm containing at least two sensors responding to different combustion phenomena and provided with a common evaluation circuit, in order to evaluate the state or condition changes of the sensors and for triggering an alarm signal. In this case the evaluation circuit, after the one sensor has been affected by a combustion phenomenon or fire, is capable of changing the response threshold of the other sensor for triggering an alarm signal in the sense of causing a sensitivity increase of such other sensor. With the heretofore known fire alarm installations there, however, does not exist any possibility of detecting the type of fire which has been encountered. In consideration of the possible selection of the fire fighting or counteractive measures which may be required this information however is frequently of decisive importance.